1. Field of the Invention
The invention relates generally to an apparatus and method for power control in a digital to analog converter (DAC) with multiple digital input channels and, more particularly, to power control in a wideband multicarrier cellular PCS basestation transmitter.
2. Description of the Related Art
A digital to analog converter (DAC) is used in cellular basestations to convert a digital carrier signal into an analog signal for transmission by the basestation transmitter. In previous designs, one DAC is provided for each digital carrier signal. In such systems, the amplitude of the input signal to the DAC must be maintained between certain limits to ensure acceptable operation of the DAC in order to meet emissions requirements specified by the applicable wireless standards.
If the amplitude of the digital signal exceeds the full scale of the DAC, the amplitude peaks of the digital signal will not be reproduced in the analog output of the DAC signal resulting in the analog signal being "clipped." This clipping by the DAC results in undesirable high frequency noise being added to the analog signal, which results in undesirable emissions being produced by the transmitter. However, if the amplitude of the digital signal is too small, the accuracy of the DAC is reduced and the signal-to-noise ratio in the DAC may be reduced to the extent that the low-level noise produced by the DAC may become a significant problem.
Thus, optimum amplitude for the digital carrier signal input to the DAC is when there is a small "backoff" such that the peak amplitude of the digital signal is slightly less than the full scale of the DAC. The amplitude of the analog signal output from the DAC is then adjusted by an analog power control circuit in order to control the output power of transmitted signal. Thus, with a single digital carrier signal input to the DAC, the optimum arrangement is have the peak amplitude of the digital signal at close to the full-scale of the DAC and to control the output power of the system by adjusting the amplitude of the analog signal output from the DAC. This prevents clipping of the signal in the DAC, thereby preventing excessive emissions, while maintaining the digital signal at a sufficiently high level to prevent noise problems in the DAC, and providing control of the output power of the system.
In basestation systems having multiple digital carrier signals, previous designs have provided multiple DACs, one for each carrier signal. This approach has been necessitated by the problems inherent in combining multiple carrier signals. Each carrier signal has a different center frequency. When the carrier signals are combined, the resultant signal will include very high peaks at those points where the peaks of the individual signals coincide. The combined digital signal will thus appear very "peaky" and have a higher peak/RMS ratio than the each of the individual signals. Furthermore, the peak/RMS ratio will increase as a greater number of individual signals are combined.
If only one DAC is used in the basestation system, the DAC receiving a combined digital signal and converting it to an analog signal, then the DAC must be designed to handle the peak amplitude of the combined digital signal, or the combined digital signal must be attenuated. Increasing the resolution of the DAC (i.e. increasing the full scale value of the DAC) would greatly increase the cost and complexity of the system. The other option is reducing the amplitude of the combined digital signal (or reducing the amplitude of each individual digital signal) so that the signal peaks do not exceed the full scale of the DAC. However, as the amplitude of the combined digital signal is reduced, the accuracy of the DAC is reduced, the signal-to-noise ratio is reduced, and the output power of the system is reduced. As the amplitude of the combined digital signal is further reduced, the noise introduced by the DAC can become very significant in comparison to each individual signal in the combined digital signal.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.